A WDM system may effectively increase transmission capacity by higher transmission speed, narrower channel spacing, or wider transmission wavelength band.
The transmission speed of the system is recently increased from 2.5 Gb/s to 10 Gb/s, and it is estimated that 40 Gb/s transmission system will emerge in the market in a few years. In order to increase transmission speed, higher optical power per each channel should be provided, and due to higher optical power, non-linearity in an optical fiber is increased to deteriorate transmission characteristics.
In a system for long-distance transmission at a transmission speed of 40 Gb/s, a Raman amplifier may be used instead of existing EDFA (Erbium Doped Fiber Amplifier) to reduce non-linearity in the optical fiber. In addition, since the limit on the signal distortion by dispersion is in the inverse relation to the square of transmission speed, if the speed is increased four times, that limit is decreased to one sixteenth. Thus, the system with a transmission speed of 40 Gb/s requires accurate dispersion compensation so that accumulated dispersion of a transmission channel does not exceed the limit. For this purpose, RDS (Relative Dispersion Slope) of a dispersion compensating fiber should be similar to RDS of an optical fiber used as a transmission line. Here, RDS is defined as dispersion slope divided by dispersion, and each characteristic is described at the same wavelength.
In order to increase transmission capacity, channel spacing of the system is narrowed from 200 GHz (1.6 nm) to 100 GHz (0.8 nm) and even to less than 50 GHz (0.4 nm). However, if the channel spacing is narrowed, signal distortion is caused due to non-linear phenomenon such as four-wave mixing or cross phase modulation. In particular, in case that dispersion is not provided sufficient to prevent a phase matching condition, crosstalk power is generated by four wave mixing, thereby distorting signals.
Crosstalk power is related to channel power and channel spacing of the system, and dispersion and effective section area of the optical fiber. If the channel power is lowered to suppress non-linearity, OSNR (Optical Signal Noise Ratio) is deteriorated to make a transmission distance shorter, thereby increasing cost of the system required for longer distance transmission.
In addition, the larger dispersion of the optical fiber lowers the crosstalk power, but the length of the dispersion compensating fiber is extended in proportion to the dispersion of the optical fiber, and the loss in the part of dispersion compensation is increased. Thus, it is needed to optimize dispersion of an optical fiber according to features of a system.
Effective section area of an optical fiber shows optical intensity per unit area. As the effective section area is larger, non-linear phenomenon is more easily suppressed.
In case of increasing transmission capacity by using additional wavelength band other than C-band (1530 to 1565 nm) and L-band (1565 to 1625 nm), using a longer wavelength than L-band is not preferred because of increase in a bending loss of an optical fiber. Thus, S-band (1460 to 1530 nm) belonging to shorter wavelength bands than C-band is preferably used. However, in this case, sufficient dispersion value is required at or near 1460 nm in order to suppress four-wave mixing in the transmission wavelength band.
In addition, if dispersion slope of an optical fiber is low, dispersion in the longer wavelength region may be decreased, thereby making it possible to extend a transmittable distance without dispersion compensation over a wide wavelength band.
Along with the changes of circumstances around WDM system, various optical fibers suitable for the changes have been proposed.
U.S. Pat. No. 5,327,516 discloses an optical fiber with the dispersion in range of 1.5 to 4 ps/nm-km at 1550 nm in order to, solve the problem that a conventional dispersion shifted fiber having dispersion approximate to zero at 1550 nm causes a large amount of four-wave mixing to deteriorate transmission characteristics. However, the optical fiber proposed in U.S. Pat. No. 5,327,516 is directed to use in a system for 360 km repeater-less transmission with transmission speed of 5 Gb/s or more, channel spacing of 1.0 to 2.0 nm and four or more channels. Thus, when the optical fiber is used in a system with channel spacing of 1.0 nm or less, non-linearity is increased, which might deteriorate transmission characteristics due to signal distortion caused by four-wave mixing or cross phase modulation.
In addition, U.S. Pat. No. 5,835,655 discloses an optical fiber with an increased effective section area more than 70 μm2 and the zero dispersion wavelength is shifted out of the transmission wavelength band to suppress four-wave mixing. The optical fiber proposed in U.S. Pat. No. 5,835,655 is more effective in restraining signal distortion caused by non-linearity due to larger effective section area. In addition, since the zero-dispersion wavelength is located in 1500 to 1540 nm or 1560 to 1600 nm wavelength region, it is possible to restrain signal distortion in C-band that is caused by four-wave mixing. However, the larger effective section area makes dispersion slope higher, and that makes a dispersion value in a long wavelength band increased. As a consequence of larger effective section area and higher dispersion slope, the transmission wavelength region without dispersion compensation is narrowed.
U.S. Pat. No. 6,396,987 discloses an optical fiber capable of decreasing dispersion compensation costs rather than a general single-mode optical fiber in a system with transmission speed of 40 Gb/s. That is to say, the optical fiber of U.S. Pat. No. 6,396,987 has dispersion of 6 to 10 ps/nm-km at 1550 nm, dispersion slope of 0.07 ps/nm2-km or less, and effective section area of 60 μm2 or more. In this case, since zero-dispersion wavelength is located near 1460 nm, i.e. Raman amplification pump wavelength, signal distortion may be caused by four-wave mixing at the pump wavelength band.